Old Irish (these are possible spellings for what is being said, as Gaelic is a very strange language when it comes to spelling): Anal nathrach, orth' bhais's bethad, do che'l de'nmha.

OR: Anáil nathrach orth bhais betha, do cheol déanta.

Modern English: Serpent's breath, charm of death and life, thy omen of making.

My translations: Anáil nathrach, ortha bhas betha, do cheol déanta.
Breath of the serpent, spell of life, the song for the maker.
Breath of serpent, spell of death and life, your song of making.

In his (incredably inaccurate) book The 21 Lessons of Merlyn, Douglas Monroe says that it is an ancient Welsh Druid spell. This is probably untrue. It is Gaelic, for one (provided the hypothesis is correct); for another, it is from a film, not from any ancient druid text--mainly because the druids didn't have texts. Their religion forbade any transcription of its dogma.